1. Field of the Invention
The present invention relates to unsaturated elastomeric AB block copolymers based on butadiene, isoprene and styrene, their manufacture and their use for the manufacture of tire treads.
2. Discussion of the Background
It is generally accepted that rubbers which are to be employed in tire making must satisfy the following requirements:
(a) The rubbers must be readily processible in subsequent tire tread manufacture. PA1 (b) The rubbers must be readily vulcanizable. PA1 (c) The rubbers must be conveniently manufactured. PA1 (d) They are required to remain highly resilient even at low temperatures. PA1 (e) They must exhibit good anti-skid properties in wet conditions. PA1 (f) They are required to have high abrasion resistance to provide a correspondingly long life expectancy. PA1 (g) When subjected to dynamic loads they should generate as little heat as possible. Their rolling resistance should be as low as possible in order to keep the fuel consumption of the vehicle as low as possible. PA1 30 to 90% of a block A, and PA1 70 to 10% of a block B, the average vinyl and isopropenyl group contents of which are each at least 30%, more particularly at least 45% higher than that of block A.
In addition, special requirements have to be complied with which arise from their particular application in tire making. It is well known that in recent times increased demands are being made on the properties of tire treads such as:
They should be either main components of tire tread blends and comply with the aforesaid requirements as such or constitute one of several components of tire tread mixtures and comply in particular with the requirement of high skid resistance without adversely affecting the rolling resistance to a noteworthy degree.
It is known that rubbers, when subjected to the torsional vibration test exhibit a temperature dependency of the logarithmic decrement of mechanical damping which when expressed as a graph, yields a graph configuration which is characteristic for the particular rubber. The desired requirements for tire treads are met in particular if the damping curve comprises a vibration damping range which is as wide as possible (See K. H. Nordsiek. Kautschuk und Gummi, Kunstoffe 38, 178 (1985) and 39, 599 (1986). This applies both to a simple rubber component and to mixtures of rubbers.
It is also known that these partly contradictory properties of tire threads are determined to a substantial extent by the nature and composition of the rubbers employed for this purpose. Homopolymers based on the conventionally employed monomeric raw materials such as butadiene, isoprene and styrene do not yet meet these requirements satisfactorily (See EP-OS 0 054 204 and JP-OS 82/87 406), nor do copolymers based on two such monomers.
For purposes of this invention, the meaning of blocks of a polymer is not restricted only to chain segments composed of different monomeric building elements, but also includes those segments which, dictated by the extraneous process parameters, exhibit abrupt variations in their nature of interlinking of the monomeric building elements.
Even if during the copolymerization of dienes and styrene, the styrene proportion is changed (See DE-0S 31 08 583) no block copolymers are attained, but merely a gradual transition. The desired improvement of tire technological properties is still inadequate, even in that case. Single phase rubber systems are described in DE-OS 31 08 583 comprising a damping maximum created by a glass transition point in a very narrow temperature range.
An improvement is attained only by virtue of a copolymer being produced comprising two different blocks A and B which differ in their structure and/or composition.
The rubber materials described in DE-PS 31 51 139 contain a styrene-butadiene block copolymer, the two blocks of which have different glass transition temperatures; the damping phase of these rubber materials exhibits a single maximum only. DE OS 32 20 680 similarly describes rubber materials which contain a block copolymer formed by a conjugated diolefin and monovinyl substituted aromatic hydrocarbons, but contains no teaching by which a wide damping curve can be attained. The rubbers do not yet satisfy the above-mentioned criteria (d) -(g) adequately.
Tire treads are described in EP-OS 0 173 791, the rubber component of which may be composed to 30 to 100% of block copolymers based on butadiene, isoprene and optionally styrene and/or piperylene. The block copolymers are produced in the presence of co-catalysts by increasing the temperature and may, for example, comprise an AB structure. The polymers always contain a terminal block based on butadiene which is formed at increased temperatures and which accordingly comprises a comparatively high content of 1,2 structural units and an uneven distribution of the vinyl groups. The rubbers described in German patent application P 37 24 871 are AB-block copolymers based on butadiene, isoprene and styrene. Both processes appear to be cumbersome, because for the manufacture of both blocks special monomer mixtures must in each case be prepared.